Double-acting axial thrust and radial bearings for grinding apparatus

ABSTRACT

A combined axial and radial thrust bearing device for a rotary shaft composed of two components, each of which comprises an annular ring fixed to the shaft and an annular ring supported free of the shaft both of which rings engage each other through bearing means. Between the two opposing free annular rings is provided a member which is adapted to exert internal pressure on the annular rings in an axial direction to minimize play between the two components. The member is adapted to be actuated by a pressure fluid in such a manner that the internal pressure exerted on the annular rings is reduced in response to an increase in pressure in an axial direction exerted on the shaft externally of the bearing.

United States Patent 1 Johansson 1 June 5, 1973 [54] DOUBLE-ACTING AXIALTHRUST AND RADIAL BEARINGS FOR GRINDING APPARATUS [75 Inventor: JohanGunnar Inge Johansson,

Taby, Sweden [73] Assignee: Defibrator Aktiebolag, Stockholm,

Sweden [22] Filed: Nov. 23, 1970 [21] Appl. No.: 91,638

[30] Foreign Application Priority Data Oct. 26, 1970 Sweden ..14436/70[52] US. Cl. ..241/37 [51] Int. Cl. ..B02c 25/00 [58] Field of Search..241/256, 37;

[56] References Cited UNITED STATES PATENTS 3,212,721 10/1965 Asplund etal. ..241/37 3,574,424 4/1971 I-lagemeister ..308/189 FOREIGN PATENTS ORAPPLICATIONS 110,804 5/1964 Czechoslovakia ..308/207 A PrimaryExaminer-Robert L. Spruill Attorney-Eric Y. Munson [57] ABSTRACT Acombined axial and radial thrust bearing device for a rotary shaftcomposed of two components, each of which comprises an annular ringfixed to the shaft and an annular ring supported free of the shaft bothof which rings engage each other through bearing means. Between the twoopposing free annular rings is provided a member which is adapted toexert internal pressure on the annular rings in an axial direction tominimize play between the two components. The member is adapted to beactuated by a pressure fluid in such a manner that the internal-pressureexerted on the annular rings is reduced in response to an increase inpressure in an axial direction exerted on the shaft externally of thebearing.

4 Claims, 5 Drawing Figures PATENTEDJUH 5197s SHEEI 20F 5 8 7M/QQ1MWPATENTEDJUH 51975 3,737,109

SHEET 4 0F 5 ii 5/ I! DOUBLE-ACTING AXIAL THRUST AND RADIAL BEARINGS FORGRINDING APPARATUS BACKGROUND OF THE INVENTION This invention relates toa device in double-acting axial thrust and radial bearings, primarilyfor grinding apparatus of the disc type.

More particularly this invention relates to a device in a double-actingaxial thrust and radial bearing, comprising two halves, each halfconsisting of an outer ring, an inner ring and, if desired, rollermembers disposed therebetween, a bearing casing element positionedbetween the two outer rings housing at least one member producing aninner pressure acting in axial direction against the outer ring membersto eliminate undesired bearing play in the two bearing halves.

The invention has been developed primarily for application in grindingapparatuses such as defibrators, refiners, preferably for workingligno-cellulose contain ing material, and it shall be exemplified in thefollowing in connection with machines of such kind. The usual type ofgrinding apparatus comprises two grinding discs rotatable relatively toone another, of which at least one is carried by a rotatable shaftsupported by at least one combined axial thrust and radial bearing, saiddisc being forced against the other disc by means of an axial, variablegrinding pressure transferred over the combined axial thrust and radialbearing.

THE PRIOR ART It is known in a combined axial thrust and radial bearingto design said means as springs, which are preloaded so that theycontinuously exert an axial inner pressure forcing the two halves of thebearing in a direction from one another while producing a radialpressure component. The object of the invention is to eliminate theeffect of the bearing play between the rotating and the stationary partsof the bearing which play otherwise would create vibrations of the shaftand destroy the parallelism of the grinding disc rotating therewith,when the grinding pressure acting in axial direction during rotation ofthe shaft falls below a predetermined value or comes to a total stop.Shorter or longer conditions of operation of this kind occurcontinuously in a grinding apparatus and the strong vibrations appearingin connection therewith have a harmful effect on the grinding apparatus.When, however, grinding pressure under supply of milling produceistransferred from the servomotor through one half of the bearing to theinterspace between the grinding disc, the bearing play disappears. Itnow becomes evident that the axial pressure from the springs exerts anadditional load on the bearing which demands a substantial reduction ofthe grinding pressure with consequent reduction of the grinding capacityin order to preserve the life of the bearing. The other alternative isto increase the dimensions of the bearing in order to reduce thespecific load on the bearing. This expedient, is, however, accompaniedby great drawbacks from the view point of space and costs.

SUMMARY OF THE INVENTION One main object of the invention is toeliminate the said inconveniences so that the members mounted in thedouble-acting axial thrust and radial bearing of the type inconsideration are fully capable to eliminate the bearing play even whenthe axial pressure acting on the bearing, primarily the grindingpressure, is insufficient for this purpose, but which are relieved whenthe interior bearing pressure, is no longer required. According to onemain feature of the invention the member is adapted to be actuated overa pressure chamber by a pressure fluid in such a manner that saidinterior pressure is reduced when axial pressure exerted from outside ofthe bearing is increased.

As an example of grinding machines to which this invention is applicablewith particular advantage the apparatus described in the co-pending US.Pat. application Ser. No. 830641 filed July 5, 1969 and now U.S. Pat.No. 3,629,482 may be mentioned, according to which the rotatablegrinding disc is forced against the other grinding disc by means of adoublesacting hydraulic servo motor, said servo motor having twopressure chambers producing an axial pressure acting on the rotatableshaft in directions opposed to one another which pressure is transferredto the grinding disc through the hearing. In a specific embodiment therotatable shaft is actuated by two servo motors for producing thegrinding pressure between the two grinding discs.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, advantagesand features of the invention will become apparent from the followingdescription, considered in connection with the accompanying drawingswhich show an embodiment of the invention and which form part of thisspecification and of which:

FIG. 1 is a vertical longitudinal section through a grinding apparatusdevised with bearings according to the Figure also on a larger scaleshowing a pilot valve for directing the pressure fluid to variouspressure chambers formed in the grinding apparatus.

FIG. 2 shows in the same section as but in an enlarged scale an axialthrust and radial bearing constructed according to the invention andlocated on the left-hand side, also to be called the front part of theapparatus as illustrated in FIG. 1.

FIG. 3 shows a portion of said bearing in a still more enlarged scale.

FIG. 4 shows an axial thrust and radial bearing in the same section butin a larger scale than in FIG. 1 located on the right-hand side in saidFIG. 1 and therefore also called the rear part.

FIG. 5 is a graph.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1reference numeral 10 denotes the base of the apparatus in which base ashaft 12 is mounted in two double-acting axial thrust and radialbearings generally denoted l4, 16. The shaft 12 is at its right-hand endformed for attachment to a driving motor not shown. The shaft carries agrinding disc 18 and is together with said grinding disc adjustable inaxial direction relatively to a stationary grinding disc 20. Thegrinding discs 18, 20 are encased in a grinding casing 22 and thegrinding produce is introduced through a channel 24 disposedconcentrically around the shaft and, if desired, provided with aconveyor 26 to be conducted by said conveyor in outward direction andworked by the grinding surfaces which are formed on the sides facing oneanother of the grinding discs in a manner known per se and therefore notdescribed here in more detail.

Mounted adjacent the front part bearing 14 and coaxially with the shaft12 is a servo motor generally denoted 28. This servo motor consists of apiston 30 which is axially displaceable but not rotatable within astationary casing 32. Formed in the casing 32 on either side of acentral flange 34 of the piston are pressure chambers for a pressurefluid and denoted, respectively, B and A and communicating with apressure fluid source in a manner to be described more closely below.The piston 30 is coupled together with an interior non-rotatable bearingbody 36 for the bearing 14 and is together with said body axiallydisplaceable relatively to the stationary servo motor casing 32 and anextension of this latter forming an outer bearing casing 33.

The bearing 14 (see especially FIGS. 2 and 3) is composed of two halvesof which the one comprises an inner ring 38 secured onto the shaft 12,an outer ring 40 carried by a stationary ring element 42 formed insection as a T and conical rollers 44 disposed between the two rings andcooperating with conical roller ways formed on the two rings. In thesame manner the other half of the bearing consists of an inner ring 46rigidly secured onto the shaft, an outer ring 48 and conical rollers 50.The two halves of the bearing are capable of together transferring aradial load from the shaft 12 to the rigid stationary base. Due to thefeature that the bearing halves are reversed they can in additiontransfer axial pressure from the servo motor 28to the grinding disc 18in mutually opposed directions.

In FIG. 4 the individual parts of the rear axial thrust and radialbearing 16 have been given the same reference numerals as in FIG. 2, butwith an additional head numeral 1. The T-shaped stationary bearingelement 142 is inserted into piston 54 of a servo motor 52 and formedwith a flange 55, said piston being nonrotatable but axiallydisplaceable within a stationary casing 56. This servo motor needs onlybe single-acting, e.g., it has only on its one side a pressure chamberBl, which is supplied with pressure fluid in a manner to be explained inthe following.

According to the invention the front bearing 14 is provided with anannular piston 58 which is located in a recess in one lateral surface ofthe T-shaped ring element 42, said recess in the same manner as thepiston extending around the circumference of the bearing. The piston 58has a plane surface 62 which acts on the outer ring 40 in the left-handhalf of the bearing 14. Formed behind the piston 58 is a pressurechamber AS, which through one or several conduits 64 is in communicationwith a pressure fluid source as shall be described in greater detailmore below. When pressure fluid is fed into the chamber AS, the piston58 will exert a pressure on the outer ring 40 and simultaneously anequally great counter-pressure will be produced to act against the outerring 48 of the right-hand bearing half. The result of these actions isthat the play between the various bearing parts is eliminated, so thatthe shaft is guided exactly in radial direction. As is indicated inFIGS. 2 and 3, the outer ring 40 is formed with a relatively greatradial play 66 relative the outer flange portion of the bearing element42 whereas the outer ring 48 is fitted into the flange of the element 42on the opposite side of central portion 43 of the latter.

The piston 58 may be formed with a plurality of axial bores 68, intowhich springs 70 are inserted. These springs have only such an elasticforce as to maintain the position of the various elements of the bearingrelative to one another, when the bearing is properly adjusted. Thepiston 58 is namely sealed against the cylindrical outer and innersurfaces of the recess 60 by means of O-rings 72, 74.

The bearing 16 may also be provided with a fluidactuated plunger, butthe necessity of such plunger is eliminated when, as in the presentcase, the servo motor 52 continuously exerts an axial pressure on thebearing in one and the same direction. Thus, in the shown embodimentonly a plurality of preloaded, relatively weak helical springs 70 areinterposed in circumferentially spaced relationship, as indicated aboveinto the T-shaped annular element 142 between the outer bearing rings140 and 148.

According to FIG. 1 a pilot valve 78 is formed with a cylindrical spacewithin which a plunger 80 is disposed slidably. The plunger 80 has threeannular flanges 82, 84, 86 which slidably cooperate with the portions-of reduced diameter 88, 90 of the cylindrical bore. Between theseportions with reduced diameter, chambers 92, 94, 96 are formed. Openinginto the central chamber 94 is a conduit 98 which is in communicationwith a pump 100 driven by a motor 99 and supplied with a pressure fluidsuch as oil from an oil pan 102 and which by means of an overflow valve104 maintains a constant pressure of e.g., 30 kpslcm Opening into theportion 88 of reduced diameter in the pilot valve is a conduit 106 whichthrough branch conduits 108, 110 communicates with the pressure chambersA and AS in the servo motor 28 and the bearing 14, respectively. Anotherconduit 112 extends from the portion 90 with reduced diameter in thepilot valve and has branch conduits 114, 116 opening into, respectively,the pressure chamber B of the servo motor 28 and the pressure chamber B1of the servo motor 52. The two lateral chambers 92, 96 in the pilotvalve are interconnected by a conduit 118 and connected to an outlet 120returning to the oil pan 102.

A control member (FIG. 1) has asleeve-shaped element 122 which isfixedly attached to the piston 30 and thus follows the same in the axialmotion thereof. Within the element 122 a cylinder 124 is manuallyadjustable in its axial position by means of a screw 126.

The cylinder 124 preferably houses a piston 128 which is actuated by thepressure in pressure chambers C and D defined by the end faces of saidpiston. The plunger 80 in the pilot valve is by means ofa spring 130continuously forced against the piston 128. When a pressure is producedin the chamber C, the piston 128 takes a left-hand end position in thecylinder 124 accordingto FIG. 1 which corresponds to the operativeposition of the grinding discs 18, 20. By, instead thereof, supplyingpressure fluid to the chamber D, the plunger 80 is reversed so that thegrinding discs are brought apart. The supply of pressure fluid to thechambers C and D is effected through a four-way valve in response toactuation of control buttons not shown here.

When the pilot valve 80 is in its starting or initial position thepressure fluid supplied through the conduit 98 is suitably distributedso that the full specific pressure, i.e., in the illustrated example 30kps/cm, prevails in the conduits 108, 110 and thus also in the pressurechambers A and AS whereas the specific pressure 5 is half the value,i.e., l5 kps/cm, in the conduits 114, 116 and thus also in the pressurechambers B and B1. This specific difference in the pressure is theresult of the plunger 80 assuming an axial position in which its radialflange 84 permits unobstructed flow from the chamber 94 to the conduit106 whereas it throttles the blow to the portion 90 of reduced diameterwhich communicates with the conduit 112. The initial adjustment of theplunger 80 is effected by means of the screw 126 and the piston 128assumes the abovementioned lefthand end position according to FIG. 1.The effective pressure area in the chamber A within the servo motor 28is preferably approximately one half of a combined effective area in thechambers B and B1, and since the combined pressures in the two lastmentioned cases act in opposite direction to the pressure in thefirstmentioned case, the resulting axial pressure exerted on the shaft12 becomes zero or approximately zero. The two grinding discs thusassume an initial position where no grinding produce is supplied to themso that no grinding pressure can be produced between them. This shouldresult in the front bearing 14 not being subjected to any axial load sothat no counterweight should be needed against undesired play in thebearing. This would result in, a powerful stroke-like vibration withconsequent deleterious effect on the bearing due to the inevitableunbalance of the system, especially in the grinding segments of therotating disc. According to'the invention, however, full oil pressure isin this position operative in the chamber AS of the bearing 14 andforces the outer bearing rings 40, 48 against the associated series ofrollers 44, 50 and these in turn against the inner rings 38 and 46,respectively, until the play in the bearing is entirely eliminated.

FIG. 5 shows a diagram the ordinate of which indicates in tons thepressures prevailing in the various chambers, whereas the abscissaindicates also in tons, how these pressures are changed by the grindingpressure operative between the grinding discs increasing from zero tofull value, which in the example is'assumed to be 35 tons. Pressuresacting in the direction from left to right according to FIG. 1 (positivepressures") are set out above the abscissa and pressures acting inleft-hand direction (negative pressures) below the same. The pressureoperative between the grinding discs follows the diagram line 160, whichextends from to 35 tons. This resulting line thus illustrates how thepressure in the interspace between the grinding discs is growing as moreand more grinding produce is fed into the interspace until fullpredetermined capacity has been reached. Due to the fact that thecontrol member 122, 126, 128 follows the servo motor piston 30 andtherewith the shaft 12, the plunger 80 of the pilot valve 78 will withincreasing grinding effect be displaced to the left whereby the supplyof pressure fluid to the chambers B, B1 grows from e.g., l kps/cm to themaximum value of 30 kps/cm. In return the pressure in the chambers A andAS falls from 30 kps/cm to 0. These changes in pressure are alsoillustrated in FIG. 5.

Thus, the pressure in the chamber B varies according to line 162, in thechamber A according to line 164 and in the chamber B1 according to line166. In the initial position, when the abscissa is zero, the pressure162 is operative in the chamber B and the pressure 166 in the chamberBl, which pressures both are positive" and thus tend to bring thegrinding discs nearer to one an other. These two pressures areoutweighed by the pressure 164 in the chamber A and the resultant willbe 0,

according to the line 160.

With full grinding effect between the grinding discs the pressurediagram line 162 has reached value 168, e.g., 22 tons, the curve line164 value on the abscissa, thus 0 ton, whereas the line 166 has risen tothe point 172 which in the example is 13 tons. The total pressureoperative between the grinding discs will thus be 22 0 I3 35 tonscorresponding to point 174 on line 160.

The resultant of the pressures acting against one another in thechambers A and B of the servo motor 28 according to the lines 164 and160 follow line 176. In the abovementioned initial position the pressureoperative in the direction away from the grinding disc 18 preponderatesin the chamber A so that the result will be a negative pressureaccording to point 176. When thereupon the grinding discs are loaded andthe plunger 80 of the pilot valve moves to the left viewed according toFIG. 1, the specific pressure in the chamber B is increased whereas itdecreases in the chamber A. The line 176 intersects the abscissa atpoint 176 and reaches the final pressure value according to point 168.

The pressure in the chamber AS follows line 178 in FIG. 5, i.e., it hasits highest value 178 when the grinding discs are not loaded and dropsthereafter towards 0 with increasing grinding pressure between saiddiscs. The axial inner pressure in the bearing 14 is thus reduced in thesame degree as the grinding pressure rises which insures elimination ofplay in the bearing by the pressure in the piston chamber AS, until thegrinding pressure has become sufficiently great to absorb the play. Itwill thus be apparent that the bearing never needs be loaded over theaxial grinding pressure whereby a correspondingly long operative life ofthe bearing is obtained.

Finally, a line 180 is drawn in FIG. 5 to indicate the load acting onthe right-hand bearing part 46, 48, 50 of the bearing 14 and thuscorresponds to the resultant of the total pressure graph 176 of theservo motor and applied inner variable load according to line 178. Inthe initial position when the grinding discs go empty, it is apparentthat a positive load according to point 181) acts on the bearing portionat right hand which load is constituted by the difference between thepressure within the chamber AS according to point 178 and the resultingnegative pressure from the chambers A and B according to point 176. Asthe pressure on the piston 30, 34 in the servo motor 28 is zeroaccording to point 176, the total axial pressure in the bearing portionhas grown to a value according to point 180 so that the bearing playthus continues to be eliminated. With full grinding effect, the curve180 ends of course also at the point 168. If, to the contrary, theinterior axial pressure in the bearing 14 should be constant from idleto full operation, the load on the bearing 14 according to point 168should become so much higher as corresponds to the axial pressureexerted in the chamber AS according to point 178. As a consequencethereof, thegrinding disc pressure and therewith the grinding effectought to be cut down, unless the life of the bearing would be allowed tobe reduced very much.

The annular piston 58 may be replaced by a plurality of pot-shapedpistons, which are mounted at spaced places about the circumference ofthe bearing element 42, 43 which is formed with corresponding axialcylindric bores. These bores form pressure chambers which are connectedwith the conduit 110 preferably made common to them all.

While one more or less specific embodiment of the invention has beenshown and described, it is to be understood that this is for purpose ofillustration only, and that the invention is not to bev limited thereby,but its scope is to be determined by the appended claims.

I claim:

1. A grinding apparatus for fibrous material comprising:

a. a pair of grinding discs;

b. a rotatable shaft carrying at least one of said grinding discs;

c. a thrust bearing supporting said shaft adapted to absorb axial aswell as radial thrust;

d. a bearing casing for said thrust bearing;

e. a servo-motor connected to said thrust bearing operative in responseto axial displacements of the shaft and the disc carried thereby;

f. a housing for said servo-motor having a piston adapted to reciprocatelongitudinally therein;

g. a flange on said piston dividing said housing into a first pressurechamber and a second pressure chamber;

h. said first pressure chamber being operative to produce grindingpressure between the discs;

i. said second pressure chamber being operative to release grindingpressure in response to variation in grinding pressure;

j. said thrust bearing comprising:

i. a pair of inner spaced annular members secured on said shaft;

ii. a stationary supporting member mounted in said bearing casingbetween said inner annular members;

iii. a pair of outer annular members resiliently carried by saidsupporting member;

iv. roller bearing members interposed between said inner and outerannular members;

k. a third pressure chamber between said supporting member and saidouter annular member responsive to axial displacement of said thrustbearing;

l. a pressure fluid connection between said third pressure chamber andsaid second pressure chamber, and

m. means for controlling the supply of pressure fluid to said pressurechambers in response to predetermined fluctuation in play of the shaft.

2. A grinding apparatus according to claim 1 in which said thirdpressure chamber comprises:

a. a recess in at least one side of said supporting member, and

b. a piston adapted to reciprocate longitudinally in said recess.

3. A grinding apparatus for fibrous material comprising:

a. a pair of grinding discs;

b. a rotatable shaft carrying at least one of said grinding discs;

c. a thrust bearing supporting said shaft adapted to absorb axial aswell as radial thrust; d. a bearing casing for said thrust bearing; e.rotationally stationary pressure means connected to said thrust bearingoperative in response to axial I displacements of the shaft and the disccarried thereby to transfer axial pressure through said thrust bearingto produce grinding pressure between the discs;

f. said thrust bearing comprising:

i. 'a pair of inner spaced annular members secured on said shaft;

ii. a pair of outer annular members carried by said casing;

iii. roller bearing members interposed with play between each pair ofsaid inner and outer annular members;

g. a pressure chamber disposed between said outer annular members andbeing connected to a supply of pressure fluid;

h. closure means for said chamber being movable axially against one ofthe outer annular members; and

i. means for controlling the supply of pressure fluid to said pressurechamber in response to fluctuation in said play so that said supplyincreases with in- 25 crease of play and vice versa.

4. A grinding apparatus for fibrous material comprising:

a. a pair of grinding discs;

b. a rotatable shaft carrying at least one of said grinding discs;

c. a thrust bearing supporting said shaft adapted to absorb axial aswell as radial thrust;

d. a bearing casing for said thrust bearing;

e. a servo-motor connected to said thrust bearing op erative in responseto axial displacements of the shaft and the disc carried thereby;

f. a housing for said servo-motor having a piston adapted to reciprocatelongitudinally therein;

g. a flange on said piston dividing said housing into a first pressurechamber and a second pressure chamber;

h. said first pressure chamber being operative to produce grindingpressure between'the discs;

. said second pressure chamber being operative to release grindingpressure in response to variation in grinding pressure;

j. said thrust bearing comprising: i

i. a pair of inner spaced annular members secured on said shaft;

ii. a stationary supporting member mounted in said bearing casingbetween said inner annular members;

iii. a pair of outer annular members engaging said inner annularmembers;

iv. a third pressure chamber disposed between said outer annular membersand connected to a fluid pressure source for exerting internal axialpressure between said outer annular members and being operative toincrease and decrease said internal pressure in response to fluctuationin play of the shaft.

1. A grinding apparatus for fibrous material comprising: a. a pair ofgrinding discs; b. a rotatable shaft carrying at least one of saidgrinding discs; c. a thrust bearing supporting said shaft adapted toabsorb axial as well as radial thrust; d. a bearing casing for saidthrust bearing; e. a servo-motor connected to said thrust bearingoperative in response to axial displacements of the shaft and the disccarried thereby; f. a housing for said servo-motor having a pistonadapted to reciprocate longitudinally therein; g. a flange on saidpiston dividing said housing into a first pressure chamber and a secondpressure chamber; h. said first pressure chamber being operative toproduce grinding pressure between the discs; i. said second pressurechamber being operative to release grinding pressure in response tovariation in grinding pressure; j. said thrust bearing comprising: i. apair of inner spaced annular members secured on said shaft; ii. astationary supporting member mounted in said bearing casing between saidinner annular members; iii. a pair of outer annular members resilientlycarried by said supporting member; iv. roller bearing members interposedbetween said inner and outer annular members; k. a third pressurechamber between said supporting member and said outer annular memberresponsive to axial displacement of said thrust bearing; l. a pressurefluid connection between said third pressure chamber and said secondpressure chamber, and m. means for contrOlling the supply of pressurefluid to said pressure chambers in response to predetermined fluctuationin play of the shaft.
 2. A grinding apparatus according to claim 1 inwhich said third pressure chamber comprises: a. a recess in at least oneside of said supporting member, and b. a piston adapted to reciprocatelongitudinally in said recess.
 3. A grinding apparatus for fibrousmaterial comprising: a. a pair of grinding discs; b. a rotatable shaftcarrying at least one of said grinding discs; c. a thrust bearingsupporting said shaft adapted to absorb axial as well as radial thrust;d. a bearing casing for said thrust bearing; e. rotationally stationarypressure means connected to said thrust bearing operative in response toaxial displacements of the shaft and the disc carried thereby totransfer axial pressure through said thrust bearing to produce grindingpressure between the discs; f. said thrust bearing comprising: i. a pairof inner spaced annular members secured on said shaft; ii. a pair ofouter annular members carried by said casing; iii. roller bearingmembers interposed with play between each pair of said inner and outerannular members; g. a pressure chamber disposed between said outerannular members and being connected to a supply of pressure fluid; h.closure means for said chamber being movable axially against one of theouter annular members; and i. means for controlling the supply ofpressure fluid to said pressure chamber in response to fluctuation insaid play so that said supply increases with increase of play and viceversa.
 4. A grinding apparatus for fibrous material comprising: a. apair of grinding discs; b. a rotatable shaft carrying at least one ofsaid grinding discs; c. a thrust bearing supporting said shaft adaptedto absorb axial as well as radial thrust; d. a bearing casing for saidthrust bearing; e. a servo-motor connected to said thrust bearingoperative in response to axial displacements of the shaft and the disccarried thereby; f. a housing for said servo-motor having a pistonadapted to reciprocate longitudinally therein; g. a flange on saidpiston dividing said housing into a first pressure chamber and a secondpressure chamber; h. said first pressure chamber being operative toproduce grinding pressure between the discs; i. said second pressurechamber being operative to release grinding pressure in response tovariation in grinding pressure; j. said thrust bearing comprising: i. apair of inner spaced annular members secured on said shaft; ii. astationary supporting member mounted in said bearing casing between saidinner annular members; iii. a pair of outer annular members engagingsaid inner annular members; iv. a third pressure chamber disposedbetween said outer annular members and connected to a fluid pressuresource for exerting internal axial pressure between said outer annularmembers and being operative to increase and decrease said internalpressure in response to fluctuation in play of the shaft.